1. Field of the Invention
The present invention relates to a hybrid-vehicle power train and, more particularly, to a hybrid-vehicle power train equipped with a CVT (continuously variable ratio transmission) unit.
2. Description of the Related Art
For a hybrid-vehicle which has an engine (which means as internal combustion engine or an external engine in the present specification), an motor (which means a motor or a motor-generator in the present specification) and an transmission, a power train applying an infinitely variable transmission (referred to as an IVT) in which a CVT unit is combined with a planetary gear unit is known as a related art. The IVT can shift between a torque circulation mode and an engine torque directly transmission mode.
In the related art, two types of configuration of power train are known. The two types of configuration are discriminated depending on positional relationship, regarding to a power flow path, between the motor and the IVT.
One is shown in FIG. 11, a motor (M) is disposed on the side of an input shaft of an IVT with respect to an engine (E/G) and the IVT, namely, between the engine and the IVT (see Japanese Patent Application Laid-Open No. 11-107798).
The other is shown in FIG. 12, a motor (M) is disposed on the side of an output shaft of an IVT. That is, an engine (E/G), the IVT, and the motor (M) are arranged in this order (see Japanese Patent Application Laid-Open No. 2002-139136).
In the related arts described in the aforementioned two patent publications (hereinafter referred to as the first and second related arts), a belt-type CVT unit is exemplified as the CVT unit of the IVT. As a matter of course, however, as described in a paragraph [0094] of Japanese Patent Application Laid-Open No. 11-107798, a CVT unit of another type such as a toroidal type or the like can also be employed in the power train. A toroidal type IVT is disclosed, for example, in pages 18 to 25 of “KOYO Engineering Journal” No. 161.
A configuration wherein a motor is disposed on the side of an input shaft of a transmission as in the case of the first related art will now be reviewed as to the following two cases.
(1) In the case where engine torque is assisted by the motor, since output torque of the motor is amplified by an IVT, there is an advantage in that large torque is available on the output side (i.e., on the side of an axle) of the IVT using the relatively small size motor. On the other hand, the efficiency in transmitting motor torque to the axle depends on the torque transmission efficiency of the IVT. Therefore, if the IVT exhibits low transmission efficiency, there is a disadvantage in that the motor exhibits low torque transmission efficiency as well.
(2) In the case where energy is regenerated by the motor, regenerable energy is regenerated by the motor with low efficiency in a low rotational speed range. In this case, since rotational speed at the engine side is higher than at the axle side owing to gear ratio of the IVT, there is an advantage in that energy can be regenerated with high rotational speed and high efficiency. On the other hand, however, there is also a disadvantage in that, because of the IVT power transmitting efficiency (power loss factor), the amount of motor regenerable energy is reduced from the amount of energy generated on the side of the axle.
A configuration wherein a motor is disposed on the side of an output shaft of an IVT as in the case of the second related art will now be reviewed as to the following two cases.
(1) In the case where engine torque is assisted by the motor, since motor torque is directly transmitted to the axle side, there is an advantage in that high torque transmission efficiency is achieved. On the other hand, however, there is also a disadvantage in that required torque on the axle side is directly associated with torque required of the motor, and in that the motor must be relatively large in size.
(2) In the case where energy is regenerated by the motor, there is an advantage in that the entire regenerable energy generated on the axle side can be received by the motor, irrespective of power transmission efficiency of the IVT. On the other hand, however, since rotation on the axle side is directly transmitted to the motor, there is a disadvantage in that a low rotational speed range on the axle side is directly associated with low regenerative efficiency of the motor itself, and in that energy is regenerated with low rotational speed and low efficiency.
The aforementioned advantages and disadvantages will be summarized as follows. In the configuration of the first related art, although the motor may be designed to generate a small torque, power is always transmitted through the IVT (which is generally inferior in efficiency to a staged transmission of planetary gear type), so that the overall efficiency worsens. In the configuration of the second related art, on the other hand, since the motor and the axle are directly connected without the intervention of the IVT, the overall efficiency (motor efficiency×transmission efficiency) is high, but a large torque is required of the motor.
A relationship among output, torque, and rotational speed is taken into account, a rise in rotational speed is linked with a decrease in torque for an equal output. In other words, a rise in vehicle speed is linked with a decrease in torque of the axle. Therefore, in high vehicle speed region, the second related art is deprived of its disadvantage and thus becomes advantageous. On the contrary, in low vehicle speed region, the first related art is advantageous if the vehicle runs at a low speed. As is apparent from the foregoing description, it would be ideal to selectively utilize the first related art and the second related art for a low vehicle speed and a high vehicle speed respectively. Heretofore, however, no good method of selectively utilizing them has been found.